Many of those types of instincts have a strong learning component - the timing and the basic directions, maybe the distance, may be instinctive, for instance, but finding the right spot once you get there is more flexible.

Indeed, I actually believe that the migration patterns of North Sea Cod has been changing for some time. It's possibly a result of lowering the overall age of the fish through overfishing i.e. 'population memory' lost with older fish.